A capacitance discharge type breakerless ignition system for an internal combustion engine has been practically used which comprises an ignition coil means including a primary coil and a secondary coil, a capacitance connected to the primary coil of the ignition coil means to be charged by a charging voltage, a controlled semiconductor switching device to discharge the capacitance through the primary coil, a signal source to signal the semiconductor switching device to be turned on, and an ignition plug connected to the secondary coil of the ignition coil means to spark when the capacitance is discharged through the primary coil of the ignition coil means by the semiconductor switching device. In such capacitance discharge type breakerless ignition system, in the event that the ignition plug is covered with soot or contaminated, it often fails to spark, resulting in prevention of a normal operation of the engine. As well known, in order to possibly reduce the failure of sparking of the ignition plug to ensure the ignition of the engine, it is effective that the ignition plug is sparkingly discharged with an abrupt voltage rise for a longer continuation time so that the discharge energy increases. An abrupt voltage rise applied across the ignition plug is particularly desirable where the plug is wet. Because in such case the spark may fail to occur if the voltage does not rise abruptly.
U.S. Pat. No. 3,234,430 discloses an ignition system for an internal combustion engine which is adapted to make longer the continuation time of spark discharge of an ignition plug. The ignition system disclosed in this U.S. patent comprises a main capacitance connected in parallel to a primary coil of an ignition coil means to be charged by a high tension charging source and an additional capacitance or capacitances connected through a choke coil or coils between the main capacitance and the high tension charging means and in parallel to the main capacitance. In the disclosed system, when a controlled switching device is closed at the ignition time of the engine, the charges of the main and additional capacitances are discharged through the primary coil of the ignition coil means with respective time constants different from each other. Thus, the ignition plug connected to a secondary coil of the ignition coil means has the spark discharges when the main capacitance is discharged and the additional one when the additional capacitance or capacitances are discharged, both of which spark discharges are accomplished in superposition. Accordingly, the spark of the ignition plug continues for a longer time, resulting in increased discharge energy of the ignition plug.
However, the prior art has the disadvantage that since the choke coil or coils have a large sized shape and a heavy weight and also since they are required to have a high insulation, the structure of the apparatus is complicated in its construction and large in its shape and weight with the result that it is expensive, which must be avoided for application of such system to a breakerless ignition apparatus.